The present invention relates generally to a projection detecting apparatus including an array type radiation detector set for detecting projections generated by radiation. More particularly, the invention is concerned with a projection detecting apparatus which can be advantageously or profitably employed in an X-ray computer tomography system for medical use.
As the X-ray detecting apparatus for the X-ray computer tomography (CT), there has been proposed in recent years a structure which comprises scintillator elements which emit light photons in response to X-rays and photodiodes serving as photoelectric conversion elements for converting the light photons into electric signals.
In the case of the X-ray computer tomography or CT system known heretofore in the art, a large number of detectors are arrayed on and along a circular arc, whereby X-ray projections of an object under inspection are sequentially detected in the different directions. In U.S. patent application Ser. Nos. 293,764 filed Jan. 5, 1989 in the name of the present inventors and others as joint applicants and U.S. Pat. No. 4,725,734, there is disclosed element blocks each constituted by a plurality of X-ray detector elements (hereinafter also referred to as detector elements or simply as elements) which are arrayed linearly. A plurality of such planar element blocks are disposed closely to one another along a circular arc having a center at which an X-ray source is disposed, whereby a detector array of a generally arcuate configuration as a whole is realized. With the aid of such element array, the X-ray projections of an object under inspection are detected.
A problem of the detector array constituted by a plurality of the element blocks such as mentioned above can be seen in that the characteristics of the elements located at the ends of each block frequently differ slightly from those of the other elements. In the so-called third generation X-ray computer tomography, non-uniformity in the sensitivity among the detector elements provides a cause for generation of ring artifact on the tomogram reconstituted. Consequently, when the projection detecting apparatus of the block structure mentioned above is used, frequently the ring artifacts appear at the locations on the tomogram which correspond to the interblock joints.
Parenthetically, relationship between the characteristics of the detector element itself and the ring artifact is discussed in Dennis L. Parker et al: "Design constraints in computed tomography: A theoretical review", Med. Phys. 9(4), July/August 1982, pp. 531-539.
As the attempts for reducing the ring artifacts mentioned above, there have been proposed a method for improved securing of the detector blocks (reference may be made, for example, to JP-A-64-88178) and a method for improving the inter-block joints (see, for example, JP-A-63-151886).
It has however be found that notwithstanding of the attempts and approaches mentioned above, the nonuniformity in the sensitivity among the elements which causes the ring artifact to appear can not satisfactorily be eliminated. Above all, in the case of the structure in which a plurality of planner element blocks are employed, as described previously, the detector elements are not arrayed on a circular arc but on and along the sides of a polygon in the strict sense. Accordingly, considering the detector elements located on a given one of the element blocks closer to both ends of that one block, respectively, it is apparent that the surfaces of these elements on which the X-ray are incident do not extend perpendicularly to the direction of the incident X-rays, as a result of which the sensitivity of the elements under consideration is lower in general when compared with that of those elements disposed in the vicinity of the center of the block. In this conjunction, it is further to be noted that each of the element blocks includes a semiconductor plate in which a plurality of photodiodes are implemented in the form of an array, wherein each of the photodiodes is assigned to each of the individual detector elements. Due to such configuration of the photodiode array, dark currents generated in the photodiodes located at both ends of the element block tend to become greater when compared with those located at inner sides to the former. For these reasons, it is safe to say that the detector elements located closer to the ends of each element block and among others those located at both outermost ends of each element block exhibit singularly deviation or error in the sensitivity.